ā is substituted for the final of aṣṭan before a case-ending.,
Thus अष्टाभिः, अष्टाभ्यः, अष्टानाम्, अष्टासु ॥ Why 'before a case-ending'? Observe अष्टत्वम्, अष्टता ॥ The rule of this sutra is an optional one, and we have in the alternative अष्टभिः, अष्टभ्यः ॥ The आ in the sutra indicates the individual letter आ, and not आ belonging to the general class आ ॥ For the generic आ would include the nasalised आं also, and as the letter replaced (अन्) is a nasal, the substitute would have been also nasal आँ, but it is not so. See 6.1.172 and 7.1.22. This rule applies also when the word stands at the end of a compound, as प्रियाष्टानः or प्रियाष्टौः 7.1.22. The word विभक्ति governs the subsequent sutras upto 7.2.114.,